Although millions of clinical laboratory enzyme assays are performed daily, not a single authoritative reference method or material has been accepted which assures the fundamental reliability and/or compatibility of these assays. Several national and international organizations are actively engaged in these problems, including the IFCC, AACC and NCCLS. The Clinical Chemistry Section of the New York State Department of Health as part of the evaluation and improvement phase of their clinical laboratory licensure program have studied extensively the lactic dehydrogenase (LDH) and aspartate aminotransferase determinations. Highly purified "enzyme standards" for LDH I and transaminase (AspAT) have been prepared from human red cells. We are extending our studies to investigation of (a) the behavior of the purified isoenzymes of LDH and AST in kinetic assays under optimal conditions using high purity reagent systems with the goal of establishing optimized enzyme methods, (b) using purified and defined enzyme materials to bring about interlaboratory standardization and (c) investigating enzyme kits in the hands of users. BIBLIOGRAPHIC REFERENCES: Rej, Robert and Raymond E. Vanderline. Azide as a preservative in assays of aspartate aminotransferase activity. Clin. Chem. 21, 158-161 (1975). Rej, Robert, C. F. Fasce, Jr., and Raymond E. Vanderlinde. Interlaboratory proficiency, intermethod comparison, and calibrator suitability in assay of serum aspartate aminotransferase activity. Clin. Chem. 21, 1141-1158 (1975).